Non Patent Literature 1 considers the contribution of radiation by the rigid polyurethane foam to thermal conductivity and describes that the smaller the radiation, the smaller the diameter of a cell forming a foam.
As described above, it is known that when the cell diameter can be made smaller, the radiation is decreased, and consequently, the thermal conductivity can be decreased and a foam having the excellent thermal insulation property can be easily obtained.
For example, Non Patent Literature 2 describes that, in order to decrease the cell diameter, the number of cells of the obtained rigid polyurethane foam is increased by introduction of a large amount of air during a stirring (mixing) step of a urethane starting material component to make nuclei of bubbles; in other words, the cell diameter is made small.
The present applicant previously proposed a method for decreasing a cell diameter of an obtained foam by introducing a specific amount of gas into at least one of a polyisocyanate component and a polyol component using a gas loading apparatus equipped with forced agitation blades thereby making a state in which fine bubbles are apparently uniformly dispersed (for example, a paste or a whipped cream state), and then performing a urethane forming reaction (see Patent Literature 1).